1. Field of the Invention
The present invention relates to an internal gear pump that performs suction and discharge of fluid by a structure in which an inner tooth of an outer gear meshes with an outer tooth of an inner gear.
2. Description of Related Art
Internal gear pumps have been used such as for automotive oil pumps, and include an inner gear having n outer teeth, an outer gear having n+1 inner teeth that mesh with the outer teeth, and a housing that houses the inner gear and the outer gear therein. The housing is provided with a suction mouth for drawing fluid and a discharge mouth for discharging the fluid. Various shapes have been suggested for the inner teeth of the outer gear and the outer teeth of the inner gear for purposes such as reduction of resistance.
For example, related art disclosed in Japanese Patent Application Publication No. 2003-322088 (JP 2003-322088 A) suggests that top lands and bottom lands of outer teeth are defined by cycloid curves that are generated by a circumscribed-rolling cycle of an inner gear that rotates while contacting the outer periphery of an inner gear base circle, which is a base circle of the outer teeth with a rotational axis of the inner gear as its center, and an inscribed-rolling circle of the inner gear that rotates while contacting the inner periphery of the inner gear base circle. Similarly, top lands and bottom lands of inner teeth are defined by cycloid curves that are generated by a circumscribed-rolling cycle of an outer gear that rotates while contacting the outer periphery of an outer gear base circle, which is a base circle of the inner teeth with an rotational axis of the outer gear as its center, and an inscribed-rolling circle of the outer gear that rotates while contacting the internal periphery of the outer gear base circle. As a result, sliding resistance and rattling are reduced. In addition, related art disclosed in Japanese Patent Application Publication No. 2005-36735 (JP 2005-36735 A) suggests that bottom lands of outer teeth of an inner gear are defined by a hypocycloid curve and that meshing sections between top lands and the bottom lands of the outer teeth of the inner gear are defined by an involute curve. This gives freedom in setting of a displacement amount of a rotor to increase a discharge amount.
In recent years, weight reduction and improved efficiency have been requested for each component of an automobile for the purpose of improved fuel efficiency of automobiles, etc. The size reduction sounds appropriate as an approach to the weight reduction. However, if only the size is simply reduced, a discharging capability of a pump is also reduced. The related art disclosed in JP 2003-322088 A uses the cycloid curve to determine shapes of the inner teeth and the outer teeth. However, when the cycloid curve is used, height of the teeth cannot be adjusted when the number of teeth is fixed. If the height of the teeth cannot be freely adjusted, it is impossible to reduce the size of an internal gear pump while maintaining the discharging capability. It is because the height of the teeth influences the discharging capability of the internal gear pump. The resistance reduction can be suggested as an approach to the improved efficiency. It has been known that efficiency of the internal gear pump is reduced by slippage that occurs between the outer tooth of the inner gear and the inner tooth of the outer gear. However, specific means for the improved efficiency is not suggested in the related art disclosed in JP 2005-36735 A.